In the die-casting process, molten metal (e.g.: Aluminum) is rapidly injected in a tightly closed mould under high pressures and is then allowed to solidify. Typical injection times for large castings are in the range of 0.1 seconds at pressures of 10,000 pounds per square inch. Before injection of the molten material into the mould cavity in said molding process, gas resides in the mould cavity. The mould is held tightly closed during injection of the molten material; this prevents the molten material from escaping said cavity. The tightly closed mould also prevents said resident gases from escaping the mould cavity during the injection phase. Said resident gases become trapped in the molded part; this results in porosity in the cast part reducing its quality and may cause the manufacturer to scrap the casting.
One commonly known method of minimizing trapped gases in the molded parts is to machine thin vent passageways leading from the perimeter of the mould cavity to the exterior of the mould. If the thickness of these passageways exceed approximately 0.01 inches, then the injected material will escape the mould cavity. Molten material escaping the mould cavity is a safety hazard and it is not practiced in the industry, further it may leave deposits on the mould parting plane which affects the subsequent sealing of the mould halves and compounds itself with subsequent casting cycles causing costly down-times to repair the mould sealing surfaces. And even if this venting method is used correctly (Thickness less than 0.01 inches), the available vent area is rarely sufficient to allow the gases to adequately escape within the short cavity fill times of 0.1 seconds.
Another known method of minimizing trapped gases in the molded parts is to machine a larger passageway leading from the mould cavity to a valve. This valve is also in fluid communication with the exterior of the mould. The object of this approach is to have a larger passageway to vent the resident gases more freely. The valve is kept open for as long as possible (During the injection phase) before the injected melt arrives at the valve chamber to evacuate the maximum amount of gas from the cavity of the mould. This valve must then be rapidly closed before injected material can escape through the valve exit. The longer the valve stays open before the molten material arrives, the more gas can be allowed to escape before the valve closes. There are a number of known inventions wherein a valve is used to permit evacuation of said resident gases in the injection moulding process (eg. die-casting). The valve described in U.S. Pat. No. 4,986,338 relies on a sensor triggered electrically to activate a valve mechanism pneumatically. This invention is at a disadvantage due to its slow response to close the valve. It also requires extensive trial and error to implement for each mould constructed and even in such measures, the valve may fail to close before the molten material arrives. If the valve fails to close in time, molten material will flow through the valve exit and fill the exhaust passageways with molten material which subsequently solidifies. This causes costly down time to remove the solidified material and to service the valve.
Another invention similar to this claim is outlined in U.S. Pat. No. 4,431,047 wherein a slidable plunger is used to permit (or block) fluid communication from the valve chamber to the mould exterior. Two symmetrical bypass passageways leading to the valve exit provide time to close the valve before molten material can flow through the valve exit. The position of said bypass passages requires a large valve that must be assembled at the exterior perimeter of the mould. The by-pass passageways are necessary to provide the time to close the valve before molten material arrives at the valve exit. There are two disadvantages to this arrangement: Firstly, a large valve limits the choices for where the valve can be installed without significant modification to the mould; this severely limits its effectiveness as it is vital to position the valve at the last-to-fill feature of the mould. Secondly, molten material (under high pressure) channeled to the outer perimeter of the mould makes it difficult to seal the passage leading to the valve from the mould cavity. This is due to thermal expansion of the mould center which severely compromises the ability to seal the passage leading to the outer perimeter of the mould. In the 6th embodiment disclosed (U.S. Pat. No. 4,431,047), the molten metal jet does not collide with the piston in an optimum angle for maximum momentum transfer. It requires a pressure buildup in the valve chamber to forcefully close the valve is yet another disadvantage; this requires more time to force to close the valve and cause the valve chamber to be filled with molten material.
An improved valve mechanism for venting injection molds which overcomes the disadvantages of the prior is required. Such a valve mechanism should be small, simple to construct and, most importantly, responsive to seal the mold vent as quickly as possible at the most opportune time.